The present invention provides a two-stage melt polymerization process for the production of high molecular weight polymers containing aromatic amide groups and, optionally, benzimidazole groups in the polymer chain.
High molecular weight polybenzimidazoles are polymers of high thermal stability and excellent resistance to oxidative or hydrolytic degradation, which can be formed into shaped articles such as fibers and films having very desirable physical properties. As taught by the published literature, these polymers may be prepared, for example, by melt polymerizing an aromatic tetraamine and an aromatic or heterocyclic dicarboxylic acid, ester or anhydride in a one or two stage process; see, for example H. Vogel and C. S. Marvel, Journal of Polymer Science, Vol. L, pages 511-539 (1961); and U.S. Pat. Nos. Re.26,065; 3,174,947; 3,509,108; 3,551,389; 4,312,976; 3,433,772; and 3,655,632. In particular, U.S. Pat. No. 3,551,389 discloses a two stage process for the production of aromatic polybenzimidazoles, in which the monomers are heated at a temperature above 170.degree. C. in a first stage melt polymerization zone until a foamed prepolymer is formed. The foamed prepolymer is cooled, pulverized, and introduced into a second stage polymerization zone where it is heated again to yield a polybenzimidazole polymer product.
In addition to the melt polymerization process described in the foregoing references, a process of preparing polybenzimidazoles has been proposed in which good results are reported to be obtained by means of the solution polymerization of an inorganic acid salt of an aromatic tetraamine and a dicarboxylic acid or a derivative thereof such as the dimethyl ester with heat in the presence of polyphosphoric acid; (see U.S. Pat. No. 3,313,783 and Iwakura et al, Journal of Polymer Science: Part A, Vol. 2, pages 2605-2515 [1964]). According to this process, the polymer product after completion of the reaction can be separated by pouring the reaction mixture in the form of a polyphosphoric acid solution into a large quantity of water. However, this separation procedure is complicated and, moreover, it is difficult to recover and reuse the polyphosphoric acid.
Copending application Ser. No. 645,952, filed Aug. 31, 1984, which is one of the parents of this application, discloses and claims a two stage process for the production of polybenzimidazoles using a free dicarboxylic acid as the dicarboxylic monomer component. Copending application Ser. No. 704,227, filed Feb. 22, 1985 which is the other parent of this application, discloses and claims a two stage process for the production of polybenzimidoles by reacting an aromatic tetraamine and a dicarboxylic component wherein no more than 95% of the carboxylate groups are the free acid or phenyl ester.
While the foregoing processes may be used to produce polybenzimidazoles of generally satisfactory properties, they require a full stoichiometric amount of a tetraamine such as 3,3',4,4'-tetraaminobiphenyl which is expensive to produce and thus raises the cost of the final polymer. Furthermore, there exist end-uses for polybenzimidazoles where, in addition to its excellent high temperature and comfort properties, some improvement in mechanical properties such as stiffness and tenacity would be highly desirable. In view of these factors, any expedient in the production of polybenzimidazoles which would have the effect of reducing the quantity of aromatic tetraamine necessary for such production and/or improve the mechanical properties of the polymer without undue sacrifice of its thermal or other desirable properties would be very beneficial.
Another class of polymers known in the art having a desirable spectrum of properties are the substantially wholly aromatic polyamides, i.e. the so-called aramids produced by condensing an aromatic diamine or derivative with an aromatic dicarboxylic acid or derivative. Some typical members of this class of polymers are disclosed, for example, in U.S. Pat. Nos. 3,063,966 of Kwolek et al and 3,094,511 of Hill et al, the entire disclosures of which are incorporated by reference. In particular, these polyamides have excellent mechanical properties, e.g. high tenacity and modulus, particularly at relatively high temperatures due to their comparatively high melting points. However, these polymers have been difficult and expensive to prepare, generally requiring the condensation of the diacid halide of the aromatic dicarboxylic acid with the aromatic diamine in a suitable solvent or emulsifying or dispersing medium. The disadvantages of this method are that it requires the use of the highly active and difficult to handle diacid halide and involves the necessity of separating the polymer from the solvent, emulsifying or dispension medium. Thus, any method for the preparation of these polymers which can be carried out with the relatively easy to handle free aromatic dicarboxylic acid or ester of such acid in a melt polymerization process not requiring the use of a major amount of solvent or emulsifying or dispersing medium would be very useful.